1. Field of the Invention
The invention relates to aqueous polymer dispersions based on polychloroprene, to a process for preparing them and to their use.
2. Description of the Related Art
Contact adhesives based on polychloroprene are predominantly solvent-based adhesives, which are applied to both of the substrates to be joined (adherends) and dried. Subsequent joining of the two adherends under pressure produces a bond with a high initial strength immediately after the joining operation. Subsequent crosslinking leads to the full, final cure.
On environmental grounds there is a growing need for suitable aqueous adhesive dispersions which can be processed to give corresponding aqueous adhesive formulations. A disadvantage of this system is that following the evaporation of the water the initial strength immediately after the joining operation is much lower than in the case of solvent-based adhesives. Adequate contacting of the adherends can therefore be achieved only by thermally activating the dry adhesive film beforehand. In addition, the possibility exists, by applying the adhesive by the “spray-mix” method, of achieving a sufficient wet strength instantaneously. With this method, the adhesive and also a coagulant are conveyed separately into a spray gun, mixed in the spray jet and coagulated. This operation is time-consuming and costly and is unsatisfactory from an economic standpoint. An overview of these methods is given, for example, in “Handbook of Adhesives”, Irving Skeist, Chapman, Hall, N.Y., 3rd Ed. 1990, Part 15, page 301, R. Musch et al., Adhesives age, January 2001, page 17, “Spray-mixing adhesives based on Dispercoll®C for foam bonding”, technical information bulletin from Bayer AG, No. KA-KR-0001d/01/05.96.
From the prior art it is known to use silica products for different applications. While solid SiO2 products are widely used to control rheological properties, as fillers or adsorbents, in the case of silica sols the dominant application is that of binders for various inorganic materials, polishes for semiconductors, or flocculation partners in colloid-chemical reactions. EP-A 0 332 928, for example, discloses the use of polychloroprene lattices in the presence of silica sols as an impregnating layer in the production of fire protection elements. FR-2 341 537 or FR-2 210 699 describes pyrogenic silicas in combination with polychloroprene lattices for producing flame-retardant foam finishes or for bitumen enhancement, while JP-A 06256738 describes them in combination with chloroprene-acrylic acid copolymers.